Synthesis and Chiroptical Properties of Helical Polyallenes Bearing Chiral Amide Pendants

Abstract

Two allene derivatives, L-and D-N-(1-(octylamino)-1-oxopropan-2-yl)-4-(propa-1,2-dien-1-yloxy)-benzamide (L-1 and D-1), bearing chiral amide pendants were designed and synthesized. Living polymerizations of L-1 and D-1 with allylnickel complex as a catalyst afforded poly-L-1 m and poly-D-1 m with controlled molecular weights and narrow molecular weight distributions. These polymers were found to possess a stable helical conformation with a preferred handedness in aprotic solvents on the basis of their circular dichroism (CD) spectra and specific rotation as well as computer simulation. The helical conformation of the polymers was revealed to be stabilized by elongation of the repeating unit until the degree of the polymerization reaches 80. The slightly influence of temperature on the CD spectra of poly-L-1 100 in CHCl 3 indicated the helical conformation was quite stable at least in the range of 0−55°C. Although poly-L-1 100 showed similar CD spectra in different aprotic solvents, remarkable decrease was observed upon the addition of protic solvents such as methanol due to the weakened hydrogen bonding interactions between the adjacent repeating units. The poly-L-1 100 behaves as a pHresponsive property; the helical structure of the main chain can be transformed to random coil by addition of trifluoroacetic acid to the THF solution which again switches back to helical conformation by neutralization with triethylamine. It was confirmed that the copolymerization of L-1 and D-1 obeyed the majority rule as supported by the nonlinear correlation between the enantiomeric excess of monomer 1 with the CD intensities of the generated copolymers. Atomic force microscope (AFM) and scanning electron microscope (SEM) studies revealed poly-L-1 100 self-assembled into well-defined helical fibrils with distinct handedness. ■ INTRODUCTIONMany biomacromolecules, such as DNA and proteins possess chiral secondary structure, which plays important roles in realizing marvelous biological activities in living systems. 1 In this context, the single-handed helical conformation is often found as one of the most essential higher-order structure. Inspired by the sophisticated biological helices and related unique functions, chemists have been challenged to develop artificial helical polymers and oligomers (foldamer). 2 Synthetic polymers possessing stable helical conformation in solution, like biomacromolecules, are of great interest. Because they can display optical activity sole based on their main-chain helical conformation. The purposes for these researches are not only for mimicking the structure and function of the biological helices but also for their wide applications in materials science. Although some stereoregular macromolecules can take a helical conformation in the solid state, they cannot maintain the helical conformation in solution because of either the inversion of the helix or a change in the structure to random coils. A stable helical conformation requires a backbone that is sufficiently rigid to restrict the rotation. Th...

Polymeric Tubular Aromatic Amide Helices

“…When the spectra were recorded for the mixture of P15 and guest AD0 with increased enantiomeric excess ( ee ), the CD signals were nonlinearly enhanced with the increase of the ee of the guest and 70% ee of the guest led to the maximum CD intensity. This result indicated that the induction of the helicity bias of the helical backbone obeyed the majority role . The induction of the twist‐sense bias of P15 by another seven chiral aspartate derivatives AD1 ‐ 7 was also studied.…”

Polypeptide‐b‐Poly(Phenyl Isocyanide) Hybrid Rod‐Rod Copolymers: One‐Pot Synthesis, Self‐Assembly, and Cell Imaging

“…[ 52 ] In sharp contrast to PBLG 35 -PPI(L) 50 , the PBLG 38 -PPI(D) 52 block copolymer showed a strong negative Cotton effect (a mirror image of that of PBLG 35 -PPI(L) 50 ) in the 260-475 nm region, which indicated that the PBLG possessed the same apha-helix as that in PBLG 35 . TEM images obtained for (b) PBLG 36 -PPI(L-co -D) 50 , (c) PBLG 35 -PPI(L) 50 , and (d) PBLG 36 -PPI(L-co -D) 50 copolymers dried from TCE. copolymers, while the PPI(D) segment behaved as an opposite-handed helical conformation to that of the PPI(L) segment in TCE.…”

“…These results suggest that no concentration-induced copolymer aggregation occurred in the current concentration range. To examine in more depth the effect of the chirality of L-PI and D-PI monomers on the helical sense of the resulting copolymers, a contrast experiment was performed on the synthesized PBLG 36 -PPI(L-co -D) 50 Table 1 (run 5). The SEC trace is shown in Figure S6 in the Supporting Information.…”

“…The SEC trace is shown in Figure S6 in the Supporting Information. The CD spectrum of PBLG 36 -PPI(L-co -D) 50 indicated that the polymer was CD silent at the PPI absorption region (Figure 2 a).…”

“…[ 34,35 ] Taking advantage of these advanced polymerization methodologies, a variety of block copolymers containing poly(3-hexylthiophene), poly(phenyl isocyanide), and poly(allene) blocks were readily synthesized. [36][37][38][39][40] Due to their unique helical structure, poly(phenyl isocyanide)s have been the focus of intense research efforts. [ 41,42 ] Kros et al described the one-pot synthesis of hybrid diblock copolymers composed of PBLG blocks and poly((S)-(-)-α-methylbenzyl isocyanide) segments prepared using 2,2′-bipyridylnickel(0) (1,5-cyclooctadiene) [Ni(cod)(bpy)] complex as a catalyst in a living manner.…”